Traditionally, the 副助詞（ふくじょし） (adverbial particle) こそ is used to emphasize whatever noun (or noun phrase) it is attached to. However, when combined with the 接続助詞（せつぞくじょし） (conjunction particle) から, it focuses on emphasizing the ‘reason’ for something. This structure is very similar to phrases like ‘precisely because (A)’ in English.
からこそ is an expression that is primarily used after either verbs or nouns. In the case of nouns, the noun will need to be followed by the 助動詞（じょどうし） (auxiliary verb) だ, before からこそ is added.

• 俺（おれ）は努力（どりょく）したからこそ、試験（しけん）に合格（ごうかく）できたんだよ。頭（あたま）がいいからだけじゃないよ。
  I was able to pass the test, precisely because I gave it my all. It's not only because I am smart.
• 先輩（せんぱい）に色々（いろいろ）と教（おし）えて貰（もら）ったからこそ、私（わたし）はここまでこれたのです。
  I was able to get to where I am now because of all the things you, senpai, have taught me.
• 君（きみ）だからこそできたんだよ。俺（おれ）にはまだそんな凄（すご）い事（こと）は出来（でき）ない。
  You were able to do it precisely because it was you. I can't do anything that amazing yet.
• ここは綺麗（きれい）な川（かわ）だからこそ、こんな美味（おい）しい魚（さかな）が釣（つ）れるんですよ。
  You can catch great fish here precisely because the river is so clean.

With からこそ, there may be several reasons for why something is the way it is, but こそ will show that there is a ‘foremost’ reason that should be considered before anything else.

• タナカ君（くん）が休（やす）みなのに仕事（しごと）に来（き）てくれたからこそ、準備（じゅんび）が早（はや）く終（お）わったり、締（し）め切（き）りまでに仕事（しごと）を終（お）わらせることができたんです。
  We were able to finish with the prep work and finish work by the deadline, precisely because you, Tanaka-kun, came into work even though it was your day off.